Switch python3 versions of test jobs to match Train PTI

In Train, we will use python 3.6 and 3.7 for python3 runtime
in our gate jobs [1]. This commit also adds python 3.7.

[1] https://governance.openstack.org/tc/reference/runtimes/train.html

Change-Id: Ie0f55b10077547e9842aa476caff0478068dd059

.zuul.yaml

@@ -3,8 +3,7 @@
       - openstack-cover-jobs
       - openstack-lower-constraints-jobs
       - openstack-python-jobs
-      - openstack-python36-jobs
-      - openstack-python37-jobs
+      - openstack-python3-train-jobs
       - publish-openstack-docs-pti
       - check-requirements
       - release-notes-jobs-python3

setup.cfg

@@ -16,6 +16,7 @@ classifier =
     Programming Language :: Python :: 2.7
     Programming Language :: Python :: 3
     Programming Language :: Python :: 3.6
+    Programming Language :: Python :: 3.7
 
 [files]
 packages = monascaclient

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-envlist = py{27,36},pypy,cover,pep8
+envlist = py{27,36,37},pypy,cover,pep8
 minversion = 2.5
 skipsdist = True